Barium getters are well known in the art. In the form of the more or less pure element barium was placed inside a metal container to protect it from reaction with the atmosphere. Then, when required to be used, it was mounted inside a vacuum device where, after partial evacuated and seal-off of the device, the barium was caused to evaporate. The barium, after evaporation, deposited in the form of a thin film within the vacuum device where it sorbed residual or unwanted gases throughout the life of the device.
While these getter devices released barium they were also found to release a large amount of unwanted gases that had been picked up during storage or handling. This was due to the getter material being barium in the form of an element which is reactive with gases.
In order to reduce the reactivity of the barium, it was then alloyed with one or more metals. Such alloys were inter alia Ba-Mg, Ba-Sr-Mg, Ba-Mg-Al. See for example the book "Getterstoff und Ihre Anwendung in the Hochvackuumtechnik" by M. Littmann. E. Winter'sche Verlagshandlung. Leipzig 1939. One of the most successful was the alloy BaAl.sub.4 having a weight percent of barium from 40 to 60 percent. Such an alloy is very inert and, as with all inert barium alloys, it must be evaporated before it can sorb gases. It can be caused to dissociate and release barium by means of applying heat to the BaAl.sub.4 alloy alone but more recently it has become widespread to mix the BaAl.sub.4 with an approximately equal weight of nickel. These two materials, in powder form, when heated react exothermically to form a solid residue of Ni-Al and evaporated Ba. However, these getter materials have to be heated to about 800.degree. C. before the exothermic reaction starts, whereupon they reach 1000.degree. C., and more, when there is the sudden release of heat on reacting exothermically.
In Japanese Patent publication number SHO 42-4123 the barium-aluminium (about 50% Ba) alloy is mixed with, preferably, 15% by weight of powdered tin to produce getters. Said getters are heated by means of high frequency at about 600.degree. C. for one minute during the exhaust process. As a result of the reaction which may be produced by said heating, it is considered that BaSn.sub.2 may be produced, or liberated barium is produced from the barium-aluminium alloy by reaction of aluminium and tin. In either case, a mixed getter material of barium-aluminium alloy and tin which is stable at a normal temperature is activated and absorbs gases at a normal temperature. Nevertheless there is a heating process involved which requires temperatures of several hundreds of degrees centigrade. Furthermore an uncontrolled chemical reaction is taking place.
Another family of getter devices has been based upon the elements zirconium or titanium. Powdered Zr 84%-Al 16%, Zr.sub.2 Fe and Zr.sub.2 Ni are among these. They are known as non-evaporable or non-evaporated getters because they do not require any of their component elements to be evaporated in order to become capable of sorbing gas. However they do require heating to a high temperature to make them gas sorptive. This is because they are covered with surface layers of oxides and nitrides which passivate them and render them inactive. Upon heating, in vacuum, the passivating layers diffuse into the bulk material and the surface becomes clean and active. This heating process usually takes place at a high temperature say 900.degree. C. for about 10-30 seconds. This temperature can be reduced but requires a longer time, for instance several hours at 500.degree. C.
Even more recently non-evaporated getters based on Zr-V have been used. Such alloys as Zr-V-Fe and Zr-V-Ni have gained widespread acceptance as "low temperature" activatable non-evaporated getters. By low temperature activatable it means that a significant proportion of their gettering activity already becomes available within a relatively short time at moderate temperatures. It is believed that this is due to the ease with which the surface layers of passivating materials may diffuse into the bulk material at these relatively low temperatures. Whatever the reason for their ability to become active at these relatively low temperatures of 400.degree.-500.degree. this can still be an undesirably high temperature under many circumstances. All these gas sorptive materials have been used in admixture with other materials, both gas sorptive or not in an attempt to lower their temperature of activation.
There are many occasions in which it is desirable to remove residual or unwanted gases from a vessel which under no circumstances can be allowed to be subjected to a high temperature. Such may be the case for instance when the vessel is made of organic plastic or contains components of organic plastic. The organic plastic may melt and even if the organic plastic does not melt it may reach such a temperature that it starts to decompose or at least give off a large amount of gas which may be hydrocarbons or other organic gases. If these are sorbed by the getter material, this causes their premature failure as they only have a finite gettering power or ability to sorb a fixed quantity of gas. The rapid sorption of a large amount of gas impairs their ability to later sorb gas during the life of the device in which they are employed. Otherwise there remains too high a gas pressure for the device to work as intended. This temperature may be as low as about 150.degree. C. At these temperatures, and lower, oxygen and water vapour permeation can be a problem.
Although it has been suggested that non-evaporated getters can be introduced into the device in a pre-activated form, that is when they have already been heated to a high temperature, they have already been subject to many manufacturing processes such as grinding to fixed particle size, mixing with other materials, compaction and forming into pellets.